There is a lot of intriguing conversation my thought has sparked. I want to reply individually to all; however, my skills in formulating coherent text are lacking, so I'll try to respond in a more general and clear manner so I don't go in circles.
1: I understand how difficult it would be to code/implement all that is required for a comprehensive space age development. I openly acknowledge I could not make this happen. However, it would be cool.
2: I don't prescribe to the idea of a linear technological development. Just because, in historical terms, we did not have proper guns till about the 15th century does not mean it was impossible beforehand. For example, the Romans did have access to both steel, steam power, and complex clockwork and gear systems. However, they never used large amounts of steel tools, powered mechanisms by steam (for any industrial purpose), and they didn't make use of clocks or other mechanical devices. This is all despite having, in some capacity, those tools. TLDR, technology is not linear (in all cases); it's more about the accumulation of knowledge and applying that knowledge in innovative ways.
3: Vintage Story does have some level of automation that is in proportion to the technology available. If one added more complex tech, one could see further automation within that same proportion of tech.
4: To my knowledge, Seraphs are functionally immortal (can't die of old age), so even if it all had to be done by hand, the passage of time would functionally not matter.
5: The game is neither strictly Stone Age nor medieval, as it has what basically amounts to magic with the gears and Rust Dimension that propelled the original humans far further technically than would be compatible with either age.
6: Vintage Story is not trying to tell a historical recreation of events from the 4th-8th century. It's trying to tell an alternative history that implements basically magic (at least this is my interpretation of how the game is).
I think that is everything I wanted to say. I do apologize if this was a thoroughly incoherent and maddening experience. I tend to only write in a technical manner (example: lab books/manuals; less literary interpretation, more solid fact).